touchpad: add a note to the synaptics touch restore feature

touchpad: add a note to the synaptics touch restore feature

We have code in place to handle the quirky transition from two to three
fingers (where one slot ends and another one starts). We do not handle the
same issue when transitioning from three to two fingers.

This is a note only because it hasn't mattered so far, at least until
eb6ef9fe70635e7c91e from #408. And it doesn't matter anymore now either
because that code is now only called for ALPS devices.

https://gitlab.freedesktop.org/libinput/libinput/issues/434#note_419912

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>